Systems and methods for power production using nested CO2 cycles

ABSTRACT

The present disclosure relates to systems and methods useful for power production. In particular, a power production cycle utilizing CO2 as a working fluid may be combined with a second cycle wherein a compressed CO2 stream from the power production cycle can be heated and expanded to produce additional power and to provide additional heating to the power production cycle.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication No. 62/212,749, filed Sep. 1, 2015, the disclosure of whichis incorporated herein by reference.

FIELD OF THE INVENTION

The present disclosure provides power production systems and methodswherein a power production cycle utilizing a CO₂ circulating fluid canbe improved in its efficiency. In particular, a compressed CO₂ streamfrom the power production cycle can be heated with an independent heatsource and expanded to produce additional power and to provideadditional heating for the power production cycle.

BACKGROUND

The most common power cycle currently employed using natural gas fuel isthe gas turbine (GT) in combination with a heat recovery steam generator(HRSG). Such system may be referred to as a natural gas fired combinedcycle (NGCC) wherein an advanced steam Rankine cycle power generationsystem (HRSG plus steam turbines) utilizes the hot turbine exhaust heatto form steam for further power generation. An NGCC unit is typicallyunderstood to be a highly efficient method of power generation utilizingpredominately natural gas fuel. In use of an NGCC unit, all CO₂, watervapor, and oxides of nitrogen (NOx) derived from combustion are ventedto the atmosphere.

Utilization of CO₂ (particularly in supercritical form) as a workingfluid in power production has been shown to be a highly efficient methodfor power production. See, for example, U.S. Pat. No. 8,596,075 to Allamet al., the disclosure being incorporated herein by reference, whichdescribes the use of a directly heated CO₂ working fluid in arecuperated oxy-fuel Brayton cycle power generation system withvirtually zero emission of any streams to the atmosphere. It haspreviously been proposed that CO₂ may be utilized as a working fluid ina closed cycle wherein the CO₂ is repeatedly compressed and expanded forpower production with intermediate heating using an indirect heatingsource and one or more heat exchangers. See, for example, U.S. Pat. No.8,783,034 to Held.

Various means have been pursued for increasing efficiency in such powerproductions methods. For example, recuperative heat exchangeroptimization has been pursued, such as via hot gas compression orthrough external heat sources. Optimization of CO₂ cycles has oftenfocused on maximizing turbine power output. Despite such efforts, thereremains a need in the field for power production systems and methodswith increased efficiency and power output while limiting orsubstantially avoiding emission of any streams (e.g., CO₂, NOx, andother combustion-related products) to the atmosphere.

SUMMARY OF THE INVENTION

The present disclosure relates to systems and methods for powerproduction wherein the efficiency of a power production cycle utilizingCO₂ as a work stream can be maximized while simultaneously increasingpower production capacity without the need for significant changes inthe equipment utilized in the power production cycle. Improvements inefficiency can be realized by supplying additional heating to theworking fluid stream beyond the heating that may be recuperated throughinternal heat exchange, the additional heating being supplied by anexternal heat source that is independent of the power production cycle.In particular, an independent heat source can be used to heat at least aportion of a high pressure recycle CO₂ stream from the power productioncycle, and the so heated stream can be rejoined to the power productioncycle in a variety of manners to achieve the additional heating of therecycle CO₂ work stream. Advantageously, the so-heated recycle CO₂stream can be expanded for additional power production and to conditionthe so-heated recycle CO₂ stream for rejoining the primary powerproduction cycle at a pressure that avoids the requirement of additionalequipment.

In some embodiments, the present disclosure thus provides a powerproduction method comprising: a first power production cycle wherein arecycled CO₂ stream is subjected to repeated compression, heating,combustion, expansion for power production, and cooling; and a secondpower production cycle wherein compressed CO₂ from the first powerproduction cycle is heated with a heat source that is independent of thefirst power production cycle, expanded for power production, andrecombined with the recycled CO₂ stream in the first power productioncycle. In particular, the heating carried out in the first powerproduction cycle upstream from the combustion can include receiving theheat that is provided to the compressed recycled CO₂ in the second powerproduction cycle. For example, the heating in the first power productioncycle can comprise passing the recycled CO₂ stream through arecuperative heat exchanger against a cooling turbine discharge stream,and the compressed CO₂ stream heated in the second power productioncycle can be passed through the recuperative heat exchanger (or aspecific segment or unit thereof) to impart additional heating to therecycled CO₂ stream in the first power production cycle. As anothernon-limiting example, the first power production cycle can include asecondary heat exchanger, and the compressed CO₂ stream heated in thesecond power production cycle can be passed through the secondary heatexchanger against a portion of the recycled CO₂ stream in the firstpower production cycle, which portion may then be recombined with theremaining recycled CO₂ stream before, during, or after passage throughthe recuperative heat exchanger.

The heat source in the second power production cycle can comprise anydevice or combination of devices configured to impart heating to astream that is sufficient to heat a compressed CO₂ stream as describedherein so that the compressed CO₂ stream achieves the desired qualityand quantity of heat. As non-limiting examples, the heat source in thesecond power production cycle can be one or more of a combustion heatsource, a solar heat source, a nuclear heat source, a geothermal heatsource, and an industrial waste heat source. The heat source may includea heat exchanger, a heat pump, a power producing device, and any furthercombination of elements (e.g., piping and the like) suitable to form,provide, or deliver the necessary heat.

In another exemplary embodiment, a method of power production accordingto the present disclosure can comprise carrying out a first cycle thatincludes: expanding a work stream comprising recycled CO₂ across a firstturbine to produce a first quantity of power; withdrawing heat from thework stream in a recuperative heat exchanger; compressing the workstream; reheating the work stream using withdrawn heat in therecuperative heat exchanger; and superheating the compressed work streamin a combustor. The method also can comprise carrying out a nested cyclewherein compressed work stream from the first cycle is heated with aheat source that is independent of the combustor and the recuperativeheat exchanger and is expanded across a second turbine to produce asecond quantity of power. In particular, the expanded work stream fromthe nested cycle can be used to add heat to the work stream in the firstcycle after the compressing and before the superheating.

In other embodiments, the present disclosure can provide methods forimproving the efficiency of a power production cycle. As a non-limitingexample, such method can comprise operating the power production cycleso that compressed, recycled CO₂ is passed through a combustor wherein acarbonaceous fuel is combusted with an oxidant to produce an exhauststream comprising recycled CO₂; the exhaust stream is expanded across aturbine to produce power and form a turbine exhaust stream comprisingrecycled CO₂; the turbine exhaust stream is cooled in a recuperativeheat exchanger; the cooled turbine exhaust stream is passed through aseparator to separate the recycled CO₂; the recycled CO₂ is compressed;and the compressed recycled CO₂ is heated by passage through therecuperative heat exchanger against the turbine exhaust stream. Suchmethod further can comprise adding further heating to the compressedrecycled CO₂ above the level of heating that is available from theturbine exhaust stream, the further heating being provided bywithdrawing a portion of the compressed recycled CO₂, heating thewithdrawn portion of compressed recycled CO₂ with a heat source that isindependent of the power production cycle, and transferring heat fromthe withdrawn and heated compressed recycled CO₂ to the remainingportion of the compressed recycled CO₂ in the power production cycle.More particularly, such method can comprise passing the withdrawn andheated compressed recycled CO₂ through the recuperative heat exchangerso as to transfer heat to the compressed recycled CO₂ therein.Alternatively, or in addition, such method can comprise passing thewithdrawn and heated compressed recycled CO₂ through a secondary heatexchanger to heat a recycled CO₂ side-stream that is thereafter combinedwith the remaining portion of the compressed recycled CO₂ in therecuperative heat exchanger. In some embodiments, such method cancomprise expanding the withdrawn and heated compressed recycled CO₂across a second turbine to produce power.

In further embodiments, the present disclosure also can provide powerproduction systems. In particular embodiments, a power production systemcan comprise: a compressor configured to compress a CO₂ stream to apressure of at least about 100 bar (10 MPa); a combustor downstream fromthe compressor; a first turbine downstream from the combustor andupstream from the compressor; a first heat exchanger positioned toreceive a stream from the compressor and to receive a separate streamfrom the turbine and configured to transfer heat between the streams; asecond turbine downstream from the compressor; and a second heatexchanger positioned to receive a stream from the compressor and toreceive a separate stream from a heat source.

In addition to the foregoing, the presently disclosed systems andmethods can be characterized in relation to one or more of thefollowing.

An external heat source (such as a gas turbine) can be integrated with apower system using CO₂ as the working fluid.

A stream derived from an external heat source (e.g., an exhaust streamfrom a gas turbine) can be cooled against a heating high pressurerecycle CO₂ stream. Optionally, the stream derived from the externalheat source can be further heated in via combustion of a carbonaceousfuel.

A high pressure recycle CO₂ stream heated by an external heat source canbe expanded in a power producing turbine. Discharge from the turbine canbe configured to correspond to an inlet, intermediate, or outletpressures of a CO₂ recycle compressor in a stand-alone power productioncycle (such as an Allam cycle described in the Example) while theturbine inlet temperature can correspond to the discharge pressure ofthe CO₂ pump in the stand-alone power production cycle.

In some embodiments, the high pressure recycle CO₂ stream heated by theexternal heat source can be heated to a temperature of about 400° C. toabout 1500° C., preferably about 700° C. to about 1300° C. The provisionof heat in such temperature range can be particularly beneficial forachieving the improvements that are described herein.

In other embodiments, an auxiliary turbine discharge flow at elevatedtemperature can be used to provide additional heat required to heat CO₂in the temperature range from ambient up to 500° C. due to the muchhigher specific heat of the CO₂ in the pressure range of about 200 bar(20 MPa) to about 400 bar (40 MPa) compared to specific heat above 500°C. Such addition of heat in a lower temperature range can bespecifically delineated from the heating provided to the high pressurerecycle CO₂ stream, as otherwise described herein. Although the additionof heat in the lower temperature range can be useful in improvingefficiency of the combustion cycle, the addition of the heat in thelower temperature range need not necessarily be combined with theaddition of heating in the greater temperature range.

If desired, additional heating of the high pressure recycle CO₂ streamsin the temperature range below 250° C. can be beneficial using heatderived from the adiabatic main air compressor of a cryogenic airseparation plant, which provides the oxygen required for the system.

The presently disclosed systems and methods are beneficial in someembodiments in that the ability is provided to combine systems such thatone or more pieces of equipment can be shared. The combination canprovide for multiple benefits, including providing for increased energyproduction and providing for reductions in capital expenditures inrelation to increased Kw capacity. Moreover, the combinations are notnecessarily limited to certain overlapping operating temperature ranges.Rather, a system operating in any temperature range may beneficially becombined with a power production cycle utilizing CO₂ as a work stream(as generally described herein) and achieve the desired improvements.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the disclosure in the foregoing general terms,reference will now be made to the accompanying drawings, which are notnecessarily drawn to scale, and wherein:

FIG. 1 is a flow diagram of an exemplary system and method of powerproduction according to the present disclosure; and

FIG. 2 is a flow diagram of a system and method of power productioncombining a gas turbine and a CO₂ cycle according to an exemplaryembodiment of the disclosure.

DETAILED DESCRIPTION

The present subject matter will now be described more fully hereinafterwith reference to exemplary embodiments thereof. These exemplaryembodiments are described so that this disclosure will be thorough andcomplete, and will fully convey the scope of the subject matter to thoseskilled in the art. Indeed, the subject matter can be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will satisfy applicable legal requirements. As usedin the specification, and in the appended claims, the singular forms“a”, “an”, “the”, include plural referents unless the context clearlydictates otherwise.

The present disclosure provides systems and methods wherein a firstpower production cycle utilizing CO₂ as a work stream can be combinedwith a second, or nested, power production cycle wherein a least aportion of the same CO₂ work stream can be subjected to additionaltreatment resulting in additional power production and/or heatproduction. In such systems and methods, high efficiencies can beachieved. In particular, recuperative heat exchange in the first powerproduction cycle can be improved while added power production can besimultaneously achieved. The additional treatment in the second powerproduction cycle can include heating with a heat source that isindependent of any heating utilized in the first power production cycle.The combination of the second power production cycle with the firstpower production cycle can be beneficial at least in part because of theability to overlap the cycles so that one or more pieces of machinerymay be utilized in both cycles. For example, a compressor utilized inthe first power production cycle can also be used as the compressor inthe second power production cycle. The present disclosure thus may becharacterized in relation to the combination of at least one directlyheated flow of CO₂ and at least one indirectly heated flow of CO₂ thatutilize shared turbo-machinery to provide at least the benefit ofincreased power output while simultaneously performing optimization of arecuperative heat exchanger. The indirectly heated flow of CO₂ can, insome embodiments, comprise at least a portion of the CO₂ from thedirectly heated flow. Thus, a single recycle CO₂ stream can be subjectto compression to form a high pressure stream as defined herein, splitinto a stream that is indirectly heated and a stream that is directlyheated, and recombined after the respective heating steps.Alternatively, a single recycle CO₂ stream can be subject to compressionto form a high pressure stream, a portion of the high pressure recycleCO₂ stream can be indirectly heated to form an indirectly heated CO₂stream, and the indirectly heated CO₂ stream can be combined with theremaining recycle CO₂ stream to form a total recycle CO₂ stream that issubject to direct heating.

In some embodiments, a high pressure stream from a first powerproduction cycle (e.g., a high pressure recycle CO₂ stream) can beheated by an independent heat source in a second power production cycle.The heated stream can then be supplied to an expander adapted for powerproduction. The expanded stream can then be inserted back to the firstpower production cycle in a variety of manners that beneficially canimpart heating to the first power production cycle beyond heating thatis available through recuperation from a cooled turbine exhaust stream.The discharge pressure from the expander in the second power productioncycle can be adapted so that the expanded stream may be inserted to thefirst power production cycle at the appropriate pressure for the pointof insertion. Heating provided to the first power production cycle inthis manner can be added in a variety of manners. For example, theexpanded stream from the second power production cycle may be useddirectly (in part or in total) as a heating stream in a recuperativeheat exchanger wherein high pressure recycle CO₂ is being re-heatedprior to entry to a combustor in the first power production cycle. As analternative example, the expanded stream from the second powerproduction cycle may be used indirectly, such as being used as a heatingstream in a further heat exchanger whereby a separate stream is heatedfor use as a heating stream in the recuperative heat exchanger.

A power production cycle useful as a first power production cycleaccording to the present disclosure can include any system and methodwherein CO₂ (particularly supercritical CO₂— or sCO₂) is used in a workstream. As a non-limiting example, U.S. Pat. No. 8,596,075 to Allam etal., which is incorporated herein by reference, describes a system andmethod wherein a recycle CO₂ stream is directly heated and used in powerproduction. Specifically, the recycle CO₂ stream is provided at hightemperature and high pressure, is provided to a combustor wherein acarbonaceous fuel is combusted in oxygen, is expanded across a turbineto produce power, is cooled in a heat exchanger, is purified to removewater and any other impurities, is pressurized, is re-heated using theheat taken from the turbine exhaust, and is again passed to thecombustor to repeat the cycle. Such system and method are beneficial inthat all fuel and combustion derived impurities, excess CO₂, and waterare removed as a liquid or a solid (e.g., ash), and there is virtuallyzero atmospheric emission of any streams. The system and method achieveshigh efficiency through, for example, the use of low temperature level(i.e., less than 500° C.) heat input after the recycle CO₂ stream hasbeen re-pressurized and before combustion.

A power production cycle useful as a first power production cycleaccording to the present disclosure can include more steps or fewersteps than described above and can generally include any cycle wherein ahigh pressure recycle CO₂ stream is expanded for power production andrecycled again for further power production. As used herein, a highpressure recycle CO₂ stream can have a pressure of at least 100 bar (10MPa), at least 200 bar (20 MPa), or at least 300 bar (30 MPa). A highpressure recycle CO₂ stream can, in some embodiments, have a pressure ofabout 100 bar (10 MPa) to about 500 bar (50 MPa), about 150 bar to about450 bar (45 MPa), or about 200 bar (20 MPa) to about 400 bar (40 MPa).Reference to a high pressure recycle CO₂ stream herein may thus be a CO₂stream at a pressure within the foregoing ranges. Such pressures alsoapply to references to other high pressure streams described herein,such as a high pressure work stream comprising CO₂.

In some embodiments, a power production method according to the presentdisclosure can comprise combining a first power production cycle with asecond power production cycle. In particular, the first power productioncycle can be a cycle wherein a recycled CO₂ stream is subjected torepeated compression, heating, combustion, expansion for powerproduction, and cooling. The second power production cycle can be acycle wherein compressed recycled CO₂ from the first power productioncycle is heated with a heat source that is independent of the firstpower production cycle, expanded for power production, and recombinedwith the recycled CO₂ stream in the first power production cycle.

As a non-limiting example, a power production system 100 and method ofuse thereof is illustrated in FIG. 1. Therein, a first power productioncycle 110 includes a combustor 115 where a carbonaceous fuel feed 112and an oxidant feed 114 are combusted in the presence of a recycle CO₂stream 143 to form a high pressure, high temperature combustion productstream 117 that is expanded in a turbine 120 to produce power with agenerator 145. The exhaust stream 122 from the turbine 120 at hightemperature is cooled in a recuperative heat exchanger 125 to produce alow pressure, low temperature CO₂ stream 127 that is passed through aseparator 130 with condensed products 132 (e.g., water) and asubstantially pure recycle CO₂ stream 133 exiting therefrom. Thesubstantially pure recycle CO₂ stream 133 is compressed in compressor135 to form a high pressure recycle CO₂ stream 137 that is split into afirst portion recycle CO₂ stream 138 and a second portion recycle CO₂stream 151. The first portion recycle CO₂ stream 138 is passed to therecuperative heat exchanger 125 where it is heated against the coolingturbine exhaust stream 122.

A second power production cycle 150 includes a heat source 160 that maybe, for example, a gas turbine that produces a high temperature, highpressure exhaust stream 162. The heated exhaust stream 162 is passedthrough a heat exchanger 155 wherein it is cooled against the heatingsecond portion recycle CO₂ stream 151 withdrawn from the first powerproduction cycle 110. Although the heat source 160 is illustrated as asingle element, it is understood that a plurality of heat sources may beused. For example two or more gas turbines may be used in parallel, or acombination of different types of heat sources (e.g., a gas turbinecombined with a waste heat source) may be used. The cooled stream 157exiting the heat exchanger 155 may be vented as illustrated. In otherembodiments, the cooled stream may be subjected to one or moretreatments. In further embodiments, the cooled stream 157 may berecycled to the heat source 160 to be again heated.

The heat source 160 may be any source adapted to provide a stream at asufficiently high temperature. In particular, the heat source may becharacterized as being independent of the first power production cycle.An independent heat source may be a heat source that is external to thepower production cycle and thus does not otherwise participate in thepower production cycle. For example, in FIG. 1, a single combustor 115is illustrated. The addition of a second combustor would be understoodto be a further heat source but would not be considered to be anexternal heat source or a heat source that is independent from the powerproduction cycle since the second combustor would directly heat therecycled CO₂ stream and the production of the heat through combustionwould directly affect the operating parameters of the further elementsof the power production cycle. As seen in FIG. 1, the heat source 160 isindependent from the first power production cycle 110 because therecycled CO₂ stream is never directly heated by the heat source 160.Rather, the heat source 160 provides heating that is indirectly added tothe recycled CO₂ stream by counter flow through the heat exchanger 155.

As non-limiting examples, the independent heat source that providesindirect heating to the recycled CO₂ stream can be one or more of acombustion heat source (e.g., a gas turbine), a solar heat source, anuclear heat source, a geothermal heat source, or an industrial wasteheat source. In further embodiments, energy may be supplied using asource that is substantially non-heating but that is combined with aheat generating element. For example, a rotating element (e.g., a windturbine) may be coupled with a heat pump.

Returning to FIG. 1, after heating in the heat exchanger 155, the heatedsecond portion recycle CO₂ stream 141 is expanded across a turbine 165to produce power with a generator 170. The turbine exhaust stream 142can be used in a variety of ways to impart further heating to the firstportion recycle CO₂ stream 138. As illustrated in FIG. 1, the turbineexhaust stream 142 is passed through the recuperative heat exchanger 125to further heat the first portion recycle CO₂ stream 138. Although theturbine exhaust stream 142 is shown entering the hot end of therecuperative heat exchanger, it is understood that the turbine exhauststream 142 may be input to the recuperative heat exchanger 125 at theappropriate heating level based upon the actual temperature of theturbine exhaust stream 142. Further, in some embodiments, the turbineexhaust stream 142 may not be returned to the heat exchanger 125.Rather, stream 142 may be input to one or both of recycle CO₂ stream 133and low temperature CO₂ stream 127. Although a single recuperative heatexchanger 125 is illustrated, a plurality of recuperative heatexchangers may be used operating at different temperature ranges, andstream 142 may be input to any one or more of said plurality ofrecuperative heat exchangers.

In other embodiments, the turbine exhaust stream 142 may be combinedwith the first portion recycle CO₂ stream 138 prior to entry to therecuperative heat exchanger 142. In such embodiments, for example,further compression may be provided to second portion recycle CO₂ stream151 and/or heated second portion recycle CO₂ stream 141.

In still further embodiments, the turbine exhaust stream 142 may passthrough a separate heat exchanger (not illustrated in FIG. 1). Firstportion recycle CO₂ stream 138 may be passed through the separate heatexchanger prior to entry to the recuperative heat exchanger. A sidestream from the first portion recycle CO₂ stream 138 taken duringpassage through the recuperative heat exchanger at an appropriateheating range may be withdrawn and passed through the separate heatexchanger, and the heated side stream can then be recombined with thefirst portion recycle CO₂ stream at an appropriate heating range. All ora portion of the heated recycle CO₂ stream 143 exiting the recuperativeheat exchanger 125 may be passed through the separate heat exchanger forfurther heating. In these exemplary embodiments, the heat provided inthe second power production cycle 150 adds further heating to the firstportion recycle CO₂ stream 138 beyond the level of heating that isavailable from the turbine exhaust stream 122 alone. The heated recycleCO₂ stream 143 is thereafter input to the combustor 115.

The turbine exhaust stream 142 from the second power production cycle150 is cooled by passage through the recuperative heat exchanger 125 andexits the cold end thereof as recycle CO₂ stream 144 which, asillustrated, is recombined with the substantially pure recycle CO₂stream 133 exiting the separator 130. Beneficially, the turbine 165 inthe second power production cycle 150 can be operated with a desiredexpansion ratio so that the pressure of the turbine exhaust stream 142is sufficiently close to a required pressure at a point in the firstpower production cycle where the recycle CO₂ stream is recombined. Insome embodiments, recycle CO₂ stream 144 exiting the recuperative heatexchanger 125 can be at a temperature such that further cooling isbeneficial. Such cooling may occur in the separator 130, for example,when the recycle CO₂ stream 144 is combined with stream 127 at a lowerpressure. Alternatively, a recycle CO₂ stream 144 may pass through anadded cooler (not shown in FIG. 1).

The additional heating provided by the second power production cycle asexemplified above can be particularly useful to reduce or eliminate thetemperature differential that otherwise exists at the hot end of therecuperative heat exchanger because of the different specific heatcapacities of the turbine exhaust entering the recuperative heatexchanger and the recycle CO₂ stream exiting the recuperative heatexchanger. Systems and methods as described herein are adapted toachieve such benefit by providing the necessary quantity and quality ofheat as the further heating. Based on the known flow rate, pressure, andtemperature of the recycle CO₂ stream entering the turbine in the secondpower production system, an expansion ratio can be chosen that allowsthe recycle CO₂ stream exiting the turbine in the second powerproduction system to provide the minimum heat quantity and temperatureneeded by the recuperative heat exchanger in the first power productioncycle.

A system and method as described above creates a thermodynamic closedloop nested within a first power production cycle. The gas mixture inthe nested cycle is, however, allowed to interact with the direct firedflow of recycle CO₂ since both cycles can share pumping equipment, aswell as condensing equipment if desired. For example, while the stream144 is shown being combined with the stream 133 in FIG. 1, the stream144 alternatively may be combined with the stream 127 prior to entry tothe separator 130 and/or prior to entry to a condenser (not illustratedin FIG. 1).

Each of the first power production cycle and the second power productioncycle may be capable of being carried out independently for powerproduction. The combination thereof, however, provides particularbenefits. In a first power production cycle such as shown in FIG. 1, anadvantage is the ability to recuperate a significant amount of the heatfrom the turbine exhaust for use in re-heating the recycle CO₂ streamafter compression and before passage to the combustor. Efficiency,however, can be limited by the ability to add enough heat to raise thetemperature of the recycle CO₂ stream exiting the hot end of therecuperative heat exchanger to be sufficiently close to the temperatureof the turbine exhaust entering the hot end of the recuperative heatexchanger. The need for input of additional heating is identified inU.S. Pat. No. 8,596,075 to Allam et al., and various possible sources oflow grade heat (e.g., at a temperature of less than about 500° C.) areidentified. The present disclosure further improves upon such systemsand methods in that an external source of heat (i.e., heat that iscompletely independent of the first power production cycle) can be usedto provide the additional heating needed to achieve the requiredrecuperator efficiency while simultaneously providing significantincreases in power generation without the need for significant changesto the primary equipment used in the first power production cycle. Inparticular embodiments, the present disclosure specifically provides forthe integration of existing power stations/equipment into a powerproduction cycle utilizing a recycle CO₂ stream as a work stream.

In some embodiments, the present systems and methods can becharacterized as being adapted for improving the efficiency of a powerproduction cycle. To this end, a power production cycle may be operatedas otherwise described herein in relation to a first power productioncycle. The power production cycle for which efficiency is improvedtypically can include any power production cycle whereby a working fluidcomprising CO₂ is repeatedly cycled at least through stages ofcompressing, heating, expansion, and cooling. In various embodiments, apower production cycle for which efficiency can be improved may includecombinations of the following steps:

-   -   combustion of a carbonaceous fuel with an oxidant in the        presence of a recycled CO₂ stream to provide a combustion        product stream at a temperature of at least about 500° C. or at        least about 700° C. (e.g., about 500° C. to about 2000° C. or        about 600° C. to about 1500° C.) and a pressure of at least        about 100 bar (10 MPa) or at least about 200 bar (20 MPa) (e.g.,        about 100 bar (10 MPa) to about 500 bar (50 MPa) or about 150        bar (15 MPa) to about 400 bar (40 MPa));    -   expansion of a high pressure recycled CO₂ stream (e.g., at a        pressure as noted above) across a turbine for power production;    -   cooling of a high temperature recycled CO₂ stream (e.g., at a        pressure as noted above), particularly of a turbine discharge        stream, in a recuperative heat exchanger;    -   condensing of one or more combustion products (e.g., water) in a        condenser, the combustion products being present particularly in        a combustion product stream that has been expanded and cooled;    -   separating water and/or further materials from CO₂ to form a        recycled CO₂ stream;    -   compressing a recycled CO₂ stream to a high pressure (e.g., a        pressure as noted above), optionally being carried out in        multiple stages with inter-cooling to increase stream density;        and    -   heating a compressed recycled CO₂ stream in a recuperative heat        exchanger, particularly heating against a cooling turbine        exhaust stream.

As noted above, improved efficiency of a power production cycleparticularly may be achieved by adding further heating to the compressedrecycled CO₂ above the level of heating (e.g., recuperative heating in aheat exchanger) that is available from a turbine exhaust stream. Thepresent disclosure achieves such further heating by utilizing a portionof the recycled CO₂ stream from the power production cycle.Advantageously, a nested cycle can be added to the power productioncycle utilizing at least the same compression equipment as used in thepower production cycle. In particularly, further heating can be providedby withdrawing a portion of the compressed recycled CO₂, heating thewithdrawn portion of compressed recycled CO₂ with a heat source that isindependent of the power production cycle, and transferring heat fromthe withdrawn and heated compressed recycled CO₂ to the remainingportion of the compressed recycled CO₂ in the power production cycle.The nested cycle thus may be substantially similar to the second powerproduction cycle described in relation to FIG. 1.

In further embodiments, the present disclosure also relates to powerproduction systems. In particular, such systems can comprise one or morepumps or compressors configured to compress a CO₂ stream to a highpressure as described herein. The systems can comprise one or morevalves or splitters configured to divide the compressed CO₂ stream intoat least a first portion CO₂ stream and a second portion CO₂ stream. Thesystems can comprise a first heat exchanger (or heat exchange unitcomprising a plurality of sections) configured to heat the first portionCO₂ stream against a high temperature turbine discharge stream and asecond heat exchanger configured to heat the second portion CO₂ streamagainst a heated stream from an external (or independent) heat source.The systems can comprise a first turbine configured to expand the firstportion CO₂ stream to produce power and a second turbine configured toexpand the second portion CO₂ stream to produce power. The systems cancomprise one or more transfer elements configured to transfer heat fromthe heated second portion CO₂ stream to the first portion CO₂ stream.The systems can comprise a combustor configured to combust acarbonaceous fuel in an oxidant in the presence of the first portion CO₂stream.

The systems of the present disclosure may be characterized in relationto a configuration as a primary power production system and a secondarypower production system, the two systems having separate heat sourcesand at least one shared compression element (and optionally at least oneshared condensing element. For example, a system according to thepresent disclosure can comprise a primary power production systemincluding a compressor configured to compress a CO₂ stream to a highpressure as described herein, a combustor downstream from thecompressor, a first turbine downstream from the combustor and upstreamfrom the compressor, and a first heat exchanger positioned to receive astream from the compressor and to receive a separate stream from theturbine. Optionally, a separator can be positioned downstream from thefirst heat exchanger and upstream from the compressor. Furtheroptionally, a compressor can be positioned upstream from the compressorand downstream from the first heat exchanger. A system according to thepresent disclosure also can comprise a secondary power production systemincluding the compressor from the primary power production system, asecond turbine downstream from the compressor, and a second heatexchanger positioned to receive a stream from the compressor and toreceive a separate stream from an external (or independent) heat source.The system can further comprise one or more valves or splittersdownstream from the compressor and upstream from each of the first heatexchanger and the second heat exchanger.

EXAMPLE

Embodiments of the present disclosure are further illustrated by thefollowing example, which is set forth to illustrate the presentlydisclosed subject matter and is not to be construed as limiting. Thefollowing describes an embodiment of a power production system andmethod utilizing a nested CO₂ cycle, as illustrated in FIG. 2.

A power production cycle was modeled based on the combination of a gasturbine with a power production cycle utilizing a circulating CO₂ workstream, such as described in U.S. Pat. No. 8,596,075 to Allam et al.,said power production cycle being referred to herein as the Allam cycle.Industrial gas turbines are efficient, low capital cost reliable systemswith a long history of technical development plus large worldwidemanufacturing capacity. The Allam cycle offers approximately the sameefficiency as the NGCC system at the same capital cost with theadvantage of capturing the whole CO₂ production from natural gas as asubstantially pure product at pipeline pressure typically between about100 bar (10 MPa) and about 200 bar (20 MPa). In the exemplaryembodiment, a gas turbine is integrated with the Allam cycle byeliminating the entire steam power system of an NGCC plant and utilizingthe hot gas turbine exhaust to provide heat for additional powergeneration using the CO₂ working fluid from the Allam cycle plusproviding the required low temperature heat input into the Allam cycleto achieve maximum efficiency. This combination allows for maintaininghigh efficiency for the integrated system while also providing lowercapital cost per Kw of installed capacity. In some embodiments, thecombination of the present disclosure can be accompanied by asubstantially insignificant drop in overall efficiency for theintegrated system. In other embodiments, however, there can besubstantially no drop in overall efficiency. In still furtherembodiments, the combination of the present disclosure can allow for anincrease in overall efficiency for the integrated system. In the variousembodiments of the present disclosure, a reduction in capitalexpenditures can also be a beneficial result.

Briefly, in the exemplary embodiment, hot exhaust from a gas turbine ispassed through a heat recovery unit similar to an HRSG which heats astream of high pressure (e.g., 300 bar (30 MPa) to 500 bar (50 MPa)) CO₂taken as additional flow from the Allam cycle CO₂ recycle compressionunits. The heated CO₂ is passed through a power producing turbine whichhas a discharge pressure corresponding to the inlet pressure of theAllam cycle CO₂ pump or to the inlet pressure or intermediate pressureof the CO₂ cycle compressor. The discharge flow from the auxiliaryturbine, which has a temperature in the range of about 200° C. to about500° C., is then used to provide the low temperature level heating forthe high pressure recycle CO₂ streams in the Allam cycle plus theadditional heating required in the gas turbine exhaust heat exchanger.Optionally there can be additional low grade heat input to the totalhigh pressure CO₂ streams by operating the cryogenic oxygen plant mainair compressor adiabatically. This releases a portion of the auxiliaryexpander discharge flow to preheat the total natural gas input to thegas turbine and Allam cycle combustors. Optionally the gas turbineexhaust can be raised in temperature with additional fuel gas firingutilizing the residual oxygen content in the gas turbine exhaust. Thisincreases the inlet temperature and power output of the auxiliary powerturbine since the high pressure CO₂ stream will be heated to a highertemperature in the gas turbine exhaust heater. Optionally the coolingflow required by the Allam cycle high pressure turbine at a temperaturein the range of about 300° C. to about 500° C. can be heated using theauxiliary turbine exhaust flow rather than the main Allam cycle turbineexhaust flow. The auxiliary gas turbine inlet temperature can be in therange of about 500° C. to about 900° C. No special internal or filmcooling or coatings for the turbine blades will be required at thesetemperatures.

An exemplary embodiment of an integrated system is shown in FIG. 2, theillustrated exemplary model being based on the integration of a GE7FBgas turbine and an Allam cycle power plant having the separateperformance characteristics shown in Table 1 below (wherein allcalculations are based on using pure methane (CH₄) as the fuel gas).

TABLE 1 7FB NGCC Allam Cycle Parameter System Power System Net PowerOutput  280.3 MW  298.2 MW Natural Gas Heat Input  488.8 MW 510.54 MWNet Efficiency 57.3% 58.41% Condenser Vacuum 1.7 inches Hg NA (0.835psia) Gas Turbine Power 183.15 MW NA O₂ Input (99.5 mol % at NA 3546MT/day 30 bar (3 MPa)) CO₂ Output (97 mol % NA 2556 MT/day purity at 150bar)

Referring to FIG. 2, a GE 7FB gas turbine 1 operating at ISO conditionshas an air input stream 64 entering the compressor of the gas turbineand a natural gas stream 3 entering the combustor 2 of the gas turbine.The gas turbine produces a 183.15 MW power output 6 from a coupledelectric generator 5. The gas turbine exhaust 4 at 624° C. can be heatedin a combustor 26 by burning an additional natural gas stream 27producing a heated stream 28 which is passed through heat exchanger 58to preheat a high pressure CO₂ recycle stream 38 at 305 bar 50° C. toproduce the heated outlet stream 29 and the cooled discharge stream 34,which may be vented. The efficiency of the overall system is not changedby burning additional fuel in the 7FB gas turbine exhaust to increasethe inlet temperature of the auxiliary high pressure turbine 7. The highpressure CO₂ recycle stream 38 is taken as an additional flow from thedischarge of the Allam cycle CO₂ pump 55, which is coupled to electricmotor 56. The turbine 7 is coupled to an electric generator 8 producingan export power stream 9. For the specific case considered the turbine 7has been specified with an outlet pressure of 30 bar (3 MPa) and aninlet pressure of 300 bar (30 MPa). The heat input to the 7FB exhaust inburner 26 is 65.7 MW. This results in the 7FB exhaust flow 4 beingheated from 624° C. to 750° C. The outlet stream 66 is at 457° C. andthe 30 bar (3 MPa) discharge pressure allows this stream, followingcooling, to be recompressed in the Allam cycle two stage recycle CO₂compressor 18 which has an inlet pressure of 29 bar (2.9 MPa). The mostfavorable outlet pressures for the turbine 7 corresponds to the inlet,intermediate, and outlet pressures for the recycle CO₂ compressor 18,which are from 29 bar (2.9 MPa) inlet to a range of 67 bar (6.7 MPa) to80 bar (8 MPa) outlet depending on cooling water/ambient coolingconditions.

The turbine outlet stream 66 is integrated into the system to preheatthe high pressure CO₂ streams in an optimum manner. Stream 66 dividesinto 3 parts. Stream 65 enters heat exchanger 68 where it is used topreheat the natural gas streams (3 a to 3, 14 a to 14 and 27 a to 27) toan outlet temperature of 425° C. and exit as stream 67. Stream 25 entersheat exchanger 60 where it is used to heat the 300 bar (30 MPa) 50° C.CO₂ stream 36 taken from the CO₂ pump 55 discharge stream 35 to producethe cooling stream 62 at 400° C. for the Allam cycle turbine 17, plusthe externally heated recycle CO₂ stream at 59 at 424° C., which entersthe main heat exchanger 61 at an intermediate point. Stream 30 entersthe 7FB exhaust cooler 58 at an intermediate point and providesadditional heating in the lower temperature section, exiting as stream32. These three separate heat exchange duties for the auxiliary gasturbine exhaust flow 66 compensate for the large increase in thespecific heat of the 300 bar (30 MPa) CO₂ stream at lower temperaturesand cover the duties required by the total heating high pressure CO₂flow.

The cryogenic air separation plant 82 produces a product oxygen stream49 at 30 bar (3 MPa) pressure and 99.5 mol % purity. The air feed stream83 is compressed adiabatically in an axial compressor 69 with a coupledbooster air compressor 70 both driven by an electric motor 71. The wholefeed air stream is compressed in 69 to 5.7 bar (0.57 MPa). The airoutlet 78 at 226° C. is used to heat an inlet 300 bar (30 MPa) CO₂stream 74 from 50° C. to 220° C. in heat exchanger 73 giving outletstream 75. This divides into two streams 76 and 77, which are introducedinto intermediate points in heat exchangers 60 and 58, respectively, toprovide further heat input at the lowest temperature level into theheating high pressure CO₂ streams 38 and 36. The main air feed stream 80and the boosted air stream 81 at 65 bar (6.5 MPa) pressure, followingcooling to near ambient temperature, enter the ASU 82.

The Allam cycle system comprises a turbine 17 with an associatedcombustor 13 coupled to an electric generator 16 producing an output 15.The natural gas fuel stream 11 is compressed to 320 bar (32 MPa) in atwo stage intercooled compressor 12 driven by an electric motor 10. Thenatural gas is preheated in 68. The turbine is directly coupled to themain CO₂ recycle compressor 18, which has two stages with an intercooler19. The inlet pressure in line 21 is 29 bar (2.9 MPa) and the dischargepressure in line 22 is 67 bar (6.7 MPa). The discharge flow 22 is cooledto near ambient temperature in heat exchanger 40 giving a CO₂ pump inletflow 39 with a density of about 0.8 kg/liter. The pump dischargeprovides (in addition to the main CO₂ recycle flow 37) additionalstreams 36, 38 and 74 used for integration of the 7FB gas turbine. Thenet CO₂ produced from the combustion of the natural gas stream 14 isdischarged at a pressure of 305 bar (30.5 MPa) as stream 84 for deliveryto a pipeline. The main recuperative heat exchanger of the Allam cycleunit 61 cools the turbine exhaust stream 24 at 725° C. to 60° C., stream41, which has stream 33 from the 7FB gas turbine integration systemadded thereto (stream 33 being a combination of stream 31 from heatexchanger 60 and stream 32 from heat exchanger 58 and stream 67 fromheat exchanger 68). The combined stream 42 is cooled near ambienttemperature in cooler 43 to produce stream 44 that enters separator 45where condensed liquid water is separated, leaving as stream 46. Theexit CO₂ gas stream 47 at 29 bar (2.9 MPa) divides into the main recycleCO₂ compressor inlet stream 21 and a stream 48 which mixes with pureoxygen stream 49 to produce an oxidant stream 50 with 25 mol % O₂content. This stream is compressed to 305 bar (30.5 MPa) in a multistagecompressor 54 (with intercooler 54 a) driven by an electric motor 52.The discharge stream 51 together with the recycle CO₂ stream 37 areheated to 715° C. in heat exchanger 61 against the turbine exhauststream 24 to form stream 20 entering the combustor 13 and stream 23entering the combustor exhaust stream to moderate the turbine 17 inlettemperature to about 1150° C.

The exemplified integrated system incorporates a specific model gasturbine which results in an efficient utilization of the heat availablein the gas turbine exhaust. Larger and smaller gas turbines, however,can be used. Performance values based on the exemplified model areprovided in Table 2.

TABLE 2 Parameter Integrated System Total Net Power Output 594.1 MWTotal Natural Gas Heat Input  1040 MW Total Net Efficiency 57.131% O₂Input (99.5 mol % at 30 bar (3 MPa)) 3546 MT/day CO₂ Output (97 mol %2556 MT/day purity at 150 bar)

The exemplified system can be used for integration of existing opencycle gas turbine units that compress ambient air as their workingfluid. It is equally applicable to the closed cycle gas turbines usingoxy-fuel combustors with the cooled turbine exhaust being used as gasturbine compressor feed following removal of produced CO₂, water inerts,and excess oxygen. For this type of gas turbine, virtually completeremoval of CO₂ from the system is possible. For a conventional opencycle gas turbine, only the CO₂ derived from the Allam cycle can beremoved for sequestration.

Many modifications and other embodiments of the presently disclosedsubject matter will come to mind to one skilled in the art to which thissubject matter pertains having the benefit of the teachings presented inthe foregoing descriptions and the associated drawings. Therefore, it isto be understood that the present disclosure is not to be limited to thespecific embodiments described herein and that modifications and otherembodiments are intended to be included within the scope of the appendedclaims. Although specific terms are employed herein, they are used in ageneric and descriptive sense only and not for purposes of limitation.

The invention claimed is:
 1. A power production method comprising:operating a first power production cycle wherein a recycled CO₂ streamis subjected to repeated compression, heating, combustion, expansion forpower production, and cooling; and operating a second power productioncycle wherein compressed recycled CO₂ from the first power productioncycle is heated with a heat source that is independent of the firstpower production cycle, expanded for power production, and recombinedwith the recycled CO₂ stream in the first power production cycle.
 2. Thepower production method of claim 1, wherein the heating in the firstpower production cycle includes receiving heat provided to the recycledCO₂ in the second power production cycle.
 3. The power production methodof claim 1, wherein the heat source in the second power production cycleis one or more of a combustion heat source, a solar heat source, anuclear heat source, a geothermal heat source, and an industrial wasteheat source.